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As time goes by, worn-out SSD should be replaced The thrown-away SSD has complex internals Other parts are still useful, only flash memories are useless High-cost Maintenance of SSD Arrays SSD Arrays Worn-out! Abandon! Replace! Dead! Live!

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I/O Services Suffered in SSD Arrays Varying data locality in an array, which consist of 80 SSDs Hot region is a group of SSDs having 10% of total data Arrays without a hot region show reasonable latency As the number of hot regions increases, the performance of SSD arrays degrades

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Why Latency Delayed? Link Contention A single data bus is shared by a group of SSDs When target SSD is ready and the shared bus is idle, the I/O request can get service right away When excessive I/Os destined to a specific group of SSDs SSD-1SSD-2SSD-3SSD-4SSD-5SSD-6SSD-7SSD-8 Dest- 3 Dest-8 Dest- 4Dest- 1Dest- 2 Dest-6 READY! IDLE!

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Why Latency Delayed? Link Contention When the shared bus is busy, even though the target SSD is ready, I/O requests should stay in the buffer This stall is because SSDs in a group share a data bus  link contention SSD-1SSD-2SSD-3SSD-4SSD-5SSD-6SSD-7SSD-8 Dest- 4Dest- 1Dest- 2 READY! Dest-6 READY! IDLE! BUSY! STALL

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Real Workload Latency (2) Prxy experiences great latency improved by Triple-A Websql did not get more benefit than expected – Due to more and hotter clusters than proxy – But, all hot clusters are located in the same switch In addition to 1) hotness, 2) balance of I/O requests among switches determines the effectiveness of Triple-A prxy websql

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Network Size Sensitivity Triple-A successfully reduces both contention time – By distributing extra load of hot clusters – Data migration and physical data reshaping Link contention time is all most completely eliminated Storage contention time is steadily reduced – It is bounded by the number of I/O requests to target clusters Normalized to non-autonomic all-flash arrays

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Why Latency Delayed? Storage Contention Regardless of array condition, independent SSDs can be busy or idle (ready to serve a new I/O) When the SSD where an I/O destined is ready, I/O can get service right away When the SSD where an I/O destined is busy, I/O should wait SSD-1SSD-2SSD-3SSD-4SSD-5SSD-6SSD-7SSD-8 Dest- 3 Dest-8 READY! BUSY!